The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Vehicles typically include an internal combustion engine that produces drive torque by combusting a mixture of air and fuel in cylinders of the engine. Combustion occurs within combustion chambers of cylinders. In spark-ignition engines, a spark plug supplies energy to the air-fuel mixture to initiate combustion. Once initiated, combustion continues along a flame front for a period.
Timing of the combustion may be generally controlled by spark timing. Spark timing may be controlled relative to a piston position and/or a crankshaft position. For example, the spark timing may be controlled relative to a top-dead-center (TDC) position of the pistons. At TDC, the volume of the combustion chamber is at its smallest volume.
Spontaneous combustion of a portion of the air-fuel mixture may occur when a pressure wave created by the spark-initiated combustion travels faster than the flame front. The pressure wave may result in a rapid pressure rise in end gases within the cylinders that causes the end gases to self-ignite (i.e., auto-ignite). Auto-ignition of the end gases may result in rapid combustion or detonation of the entire volume of end gases. Rapid combustion of the end gases results in a rapid release of heat that causes a rapid rise in cylinder pressure.
The rapid rise in cylinder pressure may cause the cylinder pressure to resonate at natural acoustic frequencies of the combustion chamber. Sustained oscillations of the pressure waves may cause metal surfaces of the combustion chamber to vibrate and produce an audible sound referred to as engine knock.
Engine control systems typically operate an engine near a knock limit for improved engine torque output and fuel economy. However, excessive engine knock can lead to undesirable audible noise and premature engine damage. Accordingly, some engine control systems include a knock detection system for detecting engine knock and initiating remedial action when engine knock is detected. Remedial action may be taken to control engine knock by lowering the knock intensity and/or inhibiting engine knock. For example, engine spark timing may be retarded to slow down the rate of combustion and thereby lower the knock intensity and/or prevent the occurrence of engine knock.
Generally, knock detection systems may be non-adaptive or adaptive. Non-adaptive knock detection systems detect engine knock based on predetermined background noise vibration. Adaptive knock detection systems detect engine knock based on background noise vibration measured during operation of the engine.
In various knock detection systems, engine knock may be detected when an intensity of the mechanical vibration within a predetermined frequency range is greater than a corresponding intensity of background noise vibration. Background noise vibration generally refers to mechanical vibration of the engine under no-knock conditions.
Several approaches have been developed to detect the occurrence of engine knock. In one approach, an accelerometer senses the mechanical vibration induced in the engine block structure as a result of the oscillating pressure wave in the combustion chamber. An energy of the mechanical vibration is used as an index of the intensity of the engine knock. In another approach, a pressure sensor senses cylinder pressure and thereby detects the oscillations in the cylinder pressure. Similar to the block structure vibration method, an energy of the pressure oscillations is used as an index of the knock intensity.